1. Field of the Invention
The present invention relates to a nonmagnetic powder for a multilayer magnetic recording medium, to a method of manufacturing the powder, and to a magnetic recording medium using the powder.
2. Description of the Prior Art
Coated type multilayer magnetic recording media are extensively utilized, having very many uses such as video tape for home use, and as such as are closely bound up with everyday life. Recently there has been a lot of research into magnetic recording media for data storage applications.
Ways of writing as much information as possible to the media include increasing the number of tape windings and increasing the density of the media themselves. The former could be achieved by reducing the thickness of the tape, while the latter could be achieved by decreasing the volume of the recording particles to thereby shrink the size the recording regions as much as possible.
With respect to achieving a higher recording density, as described in Denshi Zairyo [Electronic Materials], Vol. 35, No. 3, page 136 (Reference 1), it is necessary to carry out studies from the dual aspects of reproduction output and noise characteristics. To give a specific example, in order to improve reproduction output, it is necessary to improve the magnetizing force and decrease the recording demagnetization loss and space loss; and, to improve the noise characteristics and reduce the magnetic reversal volume, it is necessary to reduce the volume of the magnetic bodies and homogenize the particles.
The electromagnetic conversion characteristics are important for magnetic recording media. It is known that surface smoothness of the media makes a major contribution to those characteristics. With respect to decreasing the thickness of the magnetic layers, the surface properties of the nonmagnetic layer, located below the magnetic layer, has a direct effect on the surface properties of the media. JP H9-170003A (Reference 2) describes an example of previous attempts to obtain surface smoothness by specifying the shape and so forth of the particles themselves.
Concerning the smoothness of the nonmagnetic layer, through our studies, the present inventors found that while it was possible to achieve surface smoothness from the homogeneity of the particles described in Reference 1, it takes a long time to disperse the particles in the binder when preparing the nonmagnetic coating material for the lower layer.
As described above, in the case of the prior art the dispersion during the preparation of the nonmagnetic coating material takes time, so productivity is not good. In addition, if there is a lack of sufficient dispersibility, agglutination occurs during the medium coating, giving rise to surface asperities that degrade the surface smoothness of the coating. As such, improving the dispersibility improves the surface smoothness of the medium, and also helps to improve the error rate of the medium.
Therefore, an object of the present invention is to improve the surface smoothness of the medium, and to obtain nonmagnetic powder having good dispersibility in the binder, and to improve the magnetic recording medium productivity and the surface smoothness of the magnetic recording medium.
Particle shape is also important when it comes to improving the dispersibility in the binder, but it was considered that a major part is also played by the surface smoothness of the particles themselves, so the inventors carried out various studies. In these, the dehydration and baking were carried out in an ordinary atmosphere, and in the process of transforming iron oxy hydroxide to α-ferric oxide (hematite), a rapid baked densification step was introduced, accompanied by nitrogen or steam in the atmosphere, then it was found that adding the steam after completion of temperature elevation changed the surface properties of the particles, resolving the above problems.
Pore size distribution is an index of the particle surface condition. Pore size distribution of the powder particles can be prescribed by measuring the pore size distribution. In addition to using the mercury injection method proposed by this invention for the measurement, measurement can also be done by the nitrogen adsorption method. However, in the case of the nitrogen adsorption method, the smallness of the nitrogen atoms meant that the adsorption sites were also small, so that while it was possible to obtain information on micropores, it was not possible to obtain information on macropores.
The inventors focused on particle pores, investigating the effect that adjusting the pores to various sizes had on the magnetic recording medium. This yielded the result that, with respect to a pore diameter within the range 0.0018 μm to 0.1 μm, nonmagnetic powder having excellent dispersibility could be obtained when the maximum pore volume measured (calculated) by the mercury injection method was within a pore diameter range of 0.01 μm to 0.05 μm, preferably within 0.01 μm to 0.03 μm, and more preferably within the range 0.01 μm to 0.02 μm. If the maximum value came where the upper limit of 0.05 μm was exceeded, it was considered that it increases the particle surface resistance during the dispersion of the particles in the binder, reducing particle fluidity within the coating material, thereby degrading the surface smoothness of the medium. On the other hand, if the maximum value came below the lower limit of 0.01 μm, the compatibility of the particles with the binder may be poor, reducing the dispersibility.